The present invention relates to vehicle parking brakes and more specifically relates to a system for controlling spring-applied, hydraulically released parking brakes.
A known self-propelled agricultural vehicle has a pair of driven ground wheels and is equipped with a braking system including a pair of lever-actuated parking brakes respectively associated with the pair of driven ground wheels. While operators are instructed to set or engage the parking brakes when the vehicle is parked, such action is not always taken creating the hazard of the vehicle possibly rolling unattended and causing property damage and/or physical injury. Interlocks are commonly provided, however, for preventing the vehicle engine from being started unless the parking brake lever is first placed in its brake-engage position.
In order to eliminate the problem of operators forgetting or neglecting to set or engage the parking brakes when parking a vehicle, it is known to automate this function by providing vehicles with parking brakes that are spring-engaged and hydraulic pressure-released. The source of fluid pressure for disengaging these brakes is usually provided by an engine-driven pump so that any time the engine is shut down the brakes are automatically engaged. These systems also include an interlock with the vehicle transmission shift lever which results in the brakes being automatically engaged any time the shift lever is moved to its "neutral" position. Thus, the parking brakes are set any time the vehicle is parked with the transmission in neutral and the engine running. Because it is possible that the vehicle may be rolling when the transmission shift lever is placed in its "neutral" position, the known brake system includes an auxiliary ground wheel driven pump that provides sufficient fluid pressure to keep the brakes disengaged until the vehicle has slowed to a predetermined speed. Such a braking system is disclosed in U.S. Pat. No. 5,203,616 granted to Douglas Johnson on 20 April 1993.
A drawback of the patented system is that the components used for preventing brake engagement when the vehicle speed is excessive for braking add a significant cost to the system, and engagement of the brakes at even a slow speed has been found to result in jerking which is uncomfortable to the operator.